Carbon nanotubes (CNTs) are among the strongest materials ever discovered, having a tensile strength of up to 100 GPa. In order to use CNTs in practical applications, for example to weave into a cloth, it is necessary to assemble individual CNTs into macroscopic fibers. Interestingly, however, the tensile strength of CNT fibers is much lower than that of individual CNTs (e.g., less than 3.3 GPa). There exists a need, therefore, to produce CNT fibers which have increased tensile strength, and thus a wider range of practical applications.
CNT/silica composites have been extensively investigated, as silica provides stability in harsh environments, such as high temperature and strongly acidic or basic conditions, and ease of fabrication. CNT/silica composites have been produced which have a variety of morphologies, including films synthesized by solution casting, powders produced under hot pressure, xerogels produced by sol-gel technique, and nanocomposite films or powders resulting from covalent bonding between CNTs and silica. One of the main challenges for these materials is the random dispersion of CNTs in the resulting composites, which leads to much reduced mechanical strength and electrical properties. For example, CNT/silica composites with up to 30 vol. % of CNTs produced under hot pressure exhibit a tensile strength of only 85 MPa (two orders of magnitude smaller than CNT fibers). Therefore, despite the advantages of chemical stability provided by silica, it has not been apparent that silica can be used in combination with CNTs to increase mechanical strength.